The introduction of digital signal processing to hearing aids provided greater flexibility than had been previously available. In the initial excitement, many of the digital hearing aids, which often sold for $2000–$3000 each, were advertised as “CD quality.” In fact, most had only a 6–7 kHz bandwidth, and instead of the 96 dB dynamic range of compact discs, they had only 80–84 dB dynamic range. The first inventor has listened to virtually all the available digital hearing aids over the years, and found that as a result of their limited dynamic range they were unsuitable for use while playing the violin, playing the piano, directing a choir, or singing in a choir. With one or more of those activities, the digital hearing aids would distort. The distortion was sometimes so bad that the hearing aids had to be removed in order to continue with the task.
Another problem with analog hearing aids has been the degraded sound quality caused by un-damped peaks. This is also common in digital hearing aids. This can be solved with “electronic damping” as described in U.S. Pat. No. 5,812,679, for example. However, in most cases, a readily accessible switched capacitor filter design suitable for that application had a relatively high noise level. In order to overcome this noise level, a switchable pre-amplifier preceding the switched capacitor filter and a switchable attenuator following the switched capacitor filter was introduced. If the gain of the preamplifier was matched to the attenuation of the attenuator, it was possible to switch the gain in and out with no audible tick or pop, provided the switched capacitor filter was replaced by a piece of wire. When the time delay of the variable capacitor filter was introduced, a click could be heard each time the gain was automatically switched in and out as the level increased or decreased. A suitable time delay was then inserted between the time the preamplifier gain was increased, for example, and the time the post-filter attenuator was switched in. By adjusting this time delay, it was possible to minimize the click. Under no circumstances that were tested, however, was the click eliminated. This result was traced to the frequency-dependent time delay of the filter. The time-delayed switching approach worked well when the time delay of the filter block was independent of frequency and the switching delay could be made closely equal to the delay through the filter.
Up until now, the four regular players in the Chicago Symphony who wear hearing aids during performances used analog “K-AMP” hearing aids as described by Killion et al in U.S. Pat. No. 4,592,087 (1986); U.S. Pat. No. 4,170,720 (1979); U.S. Pat. No. 5,131,046 (1992); and U.S. Pat. No. 5,144,675 (1992), for the reasons described above.
Further limitations and disadvantages of conventional and traditional approaches will become apparent to one of skill in the art, through comparison of such systems with the present invention as set forth in the remainder of the present application with reference to the drawings.